RAS stands for Recirculated Aquaculture Systems, and is the generally used term in the aquaculture industry for closed fish farming plants, where a fish production plant with land based tanks is connected with a water treatment plant that cleans the water, so that a very high degree of recirculation can be achieved. Generally there is used less than 1 m3 new water per kg food in a RAS-plant, and in most plants much less, down to about 50 liter new water per kg food. More than 99% of the flow going to the fishes is typically recycled after the cleaning process.
By comparison a flow-through plant uses in the order of 50 m3 per kg food.
After discharge from the fish vessels in the cleaning plant, the water in a RAS-plant undergoes a cleaning process that normally includes removal of particular material, dissolved organic material, conversion of ammonium to nitrate, stripping of CO2 and some form of antibacterial treatment of the water, often in form of UV-radiation.
RAS-technology has a number of advantages, among other things that the water quality, and not least, the temperature can be completely controlled the entire year, which again means that one among other things can breed fish at much higher densities and higher growth rate than in traditional pond farming, where low temperatures in the winter time leads to reduced or no growth.
As the temperature control is crucial for the economy in the plants, the plants are generally constructed indoors; except eventually in the tropics where the temperature is fairly constant the entire year.
Temperature control and a high fish density are crucial for the economy in a RAS-plant.
The cold-blooded fish grow much faster when temperature control is used, and the production time can under certain circumstances be cut in half compared to outdoor plants.
The high fish density means that the plants can be reduced in size, and thereby can the building price for the plants be reduced substantially.
There are different water treatment technologies for RAS-plants.
As RAS-plants normally depends on an indoor installation, combined with an expensive cleaning plant, RAS-plants are relative expensive to install compared to other plants, and the technology has therefore so far primarily been used for production of young fish, where the supply safety and quality is more important than the production price.
The produced young fish of e.g. salmon, is normally put out in open cages at sea where they grow to slaughter size.
There is however a high interest in also being able to produce, e.g. salmon, to slaughter size on land, but today's plants are so expensive to install that it so far only has been competitive with substantial subsidies to the plant investment.
With the known technology a RAS-plant is constructed with a structure as in a traditionally land-based fish farming plant, where the fish are bread in a number of vessel/tank units. In a traditional plant water is lead to the vessel/tank units from lake, sea or river, possible by means of pumps, and the water is after use lead from the tanks out in a recipient.
In a RAS-plant the water is instead lead from the tanks via pipes or channels to a water treatment plant, where the water is treated for waste products and, to an extent, in most cases moreover goes through some kind of bacterial treatment, after which the water is lead back to the fish vessels.
A RAS-plant constructed according to the know concepts consists thus of three main components. a) a number of vessel/tank units that are connected with pipes to, b) a water treatment plant, c) and a weather screen, surrounding the vessel/tank units and the water treatment plant, often in the form of an isolated building, typically constructed as standard industrial/farming building using steal rafters. Nowadays there will typically be three main suppliers involved in the construction of a RAS-plant, a supplier of the water treatment technology, a tank supplier and a building supplier. It is furthermore characteristic of existing RAS-concepts that the piping is rather extensive, as a lot of the piping between fish vessels and water treatment is installed beneath the bottom level in the fish vessels.
Model Pond Farming
The so-called model pond farms constitute something between RAS-plants and open traditional land-based fish farming.
These plants are constructed outdoors, and treatment of the water is not so intensive as in RAS-plants, which means that recirculation of water is significantly lower and there is no temperature control. The model pond farm plants are moreover at lot more bulky than RAS-plants.
In RAS-plants there are used the same kinds of vessels that generally have been used for fish farming. These vessels can be divided in three types.
1) Round tanks with inlet in the periphery and outlet in the center.
This vessel type is characterized in that it has a good self-cleaning effect, and it is easy to create good flow conditions for the fish, provided that it is a fish type that can endure a certain flow speed. The round tank is particular optimal for salmon fish that grow and thrive optimal at a high water speed.
It is a simple and strong construction, and is in RAS-plants the most used type of tanks. The main disadvantage is that this construction type is bulky, as there is a lot of space being wasted between the tanks. This does not mean a great deal in traditional outdoor plants, but it is a problem when the plants are to be established under roof at a high square meter price.
In order to optimize the space one has, especially in RAS-plants, often modified the round tank to an octagon tank, but basically with the same function and the same properties as the round tank. The tank is either serviced from the side (requires additional space) or via a top mounted crossing.
Only one group of fish can be present in each tank.
2) D-ended raceways, is a elongated round vessel, which consists of two half circles, where the outer wall of the two half circles are connected to each other with two parallel upright tank walls, likewise is the center for each of the two half circles connected to each other. The water is normally brought into the periphery, with outlet at both ends of the center wall. Hereby a circular flow is established similar with the round tank.
The advantage of a D-ended raceway is that it utilizes the space better, but it is not self-cleaning to the same extent, and the construction of the tank is relative expensive, since it is not possible to utilize the strength of the circle in the same way as it is for the round tank, and at the same time the water pressure results in a substantial moment on the long sides of the tank. In practice it has often turned out to be difficult to make this kind of tank to function optimally.
The tank is either serviced from the side (demands space) or via a top mounted crossing.
Normally only one group of fish is present in each tank, but theoretically the tank could be divided into several sections.
3) Raceway/longitudinal flow vessel/channel tanks have been used for several hundreds of years. This tank type is also used, though not often, in RAS-plants. The tank type utilized the space better, but the flow and the self-cleaning properties are not optimal.
The tank type constitutes a rectangular box with two parallel upright long sides, and two parallel end sections. The bottom is normally the same in the entire tank, but the bottom level can if desired fall towards the outlet end of the tank.
A longitudinal flow vessel has water inlet in one end, and outlet in the other, thereby the flow differentiates itself somewhat from the flow in the two above-mentioned vessel types.
In the round tank and in the D-end tank a mixing of the new water and the water already present in the tank occurs, so that the water quality is almost uniform in the entire tank.
In a longitudinal flow vessel cleaned water is lead in at one end and the dirty water out at the other end, so that there is a gradient in the tank, where the water becomes more impure the closer it gets to the outlet end.
The water speed will be low in a raceway, where the water in principle just runs from one end to the other, contrary to round vessels or a D-ended tank, where the water will circulate round in the tank many times before it runs out of the vessel again. The low water speed and lack of centrifugal force means that a longitudinal flow vessel can be difficult to keep clean.
A longitudinal flow vessel has the advantage, although rarely used, that it can be divided into sections, so that several groups of fish can be present in the same tank.
In traditional plants longitudinal flow vessels are normally serviced from the longitudinal side, whereas crossings normally are installed over the tanks in RAS-plants, in order to reduce the building area.
In the model pond farming the tanks are serviced from the side as in traditional pond farms.
Where there in RAS-plants in most cases is used round tanks, then there are most often used tanks of the longitudinal flow type in plants constructed after the model pond farming concept. In the model pond farms the water is lead from water treatment plants through an inlet arrangement to a set-up of several parallel longitudinal vessels. At the outlet end of these longitudinal vessels the outlet water is collected in a channel system in order to be lead back to the water treatment plant.
In pond farming alternatively two large parallel longitudinal flow vessels is used, which are connected at one end, so that they have outlet and inlet at opposite ends, and the outlet from one longitudinal flow vessel lead directly over and functions as inlet to the next longitudinal flow vessel, the tanks are in this way connected in series.
The outlet from the last longitudinal flow vessel lead then directly 100% over in a water treatment system. After the treatment the water is again lead to the first longitudinal flow vessel. The flow in the tank is in this way limited by the water amount that is lead into the tank from the water treatment system.
Several connected longitudinal flow vessels, corresponds to only one long longitudinal flow vessel that are bend double at the center, and where there is installed a water treatment plant between inlet and outlet.
In model pond farming with huge longitudinal flow vessels, these will normally be divided into several sections, so that several groups of fish can be kept.
However, it has turned out that there are some disadvantages with the known prior art, reviewed shortly hereunder.
Space requirements. There are two conditions that especially mean that the plants require a lot of space. One is that there is a huge waste of space around the tanks, especially with the round vessels, which by far is the preferred vessel type in RAS-plants, since it is possible with these vessels to maintain a good water quality, even at the high fish densities that are provided in RAS-plants.
The other condition that causes the plants to take up much space is that the average fish density in the individual vessels are substantially below the fish density that one can operate with. This is due to the fact that when fish are moved to a new vessel, then plans are made such that the fish for example can double their weight, before the vessel is again emptied of fish. From the fish are placed in the vessel until the vessel again is emptied one does not have the option of changing the vessel volume. The fish density in the vessel will therefore, most of the time, be below what is economically optimal, and in practice the necessary tank volume is therefore much higher than what is required theoretically if one consistently could operate at optimal fish density.
Extensive piping. The construction of a RAS-plant, after the known concepts, include a very extensive piping, which takes time and constitute a substantially part of the investment funds.
Vessel types. When choosing fish vessels in existing RAS-plants one has had a choice between a) round vessels that take up much space and that are expensive in piping, but that are well suited for breeding fish at high density, or choice of b) longitudinal flow vessels that uses the area better, and where less piping is required, but that costs more to construct, and where it for several fish species is not possible to create the optimal water speed, hereunder salmon. When using longitudinal flow vessels it is likewise difficult or inappropriately expensive to ensure a water quality that meets the demands for fish breeding.
Building time. It takes a long time to construct a RAS-plant after the present concepts, typically 9-12 months; which, especially, is due to the comprehensive piping beneath the bottom level of the tanks.
From DE2829496 is known a plant for breeding water animals, that reduces the investment funds, by having an outer circular tank with several circular tanks one within the other, where only the outermost wall can handle a one-sided water pressure, which is the reason why all vessels are connected near the bottom level to accommodate one-sided water pressure. Outermost annular tank is for breeding of water animals and divided in preferably 3 smaller sections that all are individually connected to water treatment through the tank bottom. The outer supporting wall, and the uniform water pressure in the plant, means that the walls between the central tank and the surrounding tanks can be constructed thinner than in previous known design, since the walls become exposed to uniform water pressure from both sides. At the same time the necessary building area is reduced by having water treatment in the innermost rings. In the outermost annular tank it is possible to move the division between the three sections, although limited by the fixed installed outlets through the bottom of the tank.
However, it has turned out that there are some disadvantages with this technique, first of all the concept results in an inappropriate flow structure that mainly is vertical, where the fish that primarily are desired to be breed with the present invention, depend on a horizontal/laminar flow structure. Moreover, the connection between each annular tank results in that the entire plant must be shut down, including the water treatment, if a single tank are to be emptied. The plant uses moreover comprehensive pipe installations beneath the level of the tanks, as known from previous designs; the tank concept is basically constructed conventionally, where each individual tank section basically is an isolated tank with its own inlet—an outlet, but having a shape that prevents the establishment of a horizontal flow structure suitable for breeding of certain species, including kingfish and salmon fish.
DE2829496 discloses a fish farming plant as described in the introduction. The fish farming plant does not comprise a biofilter and does not comprise section walls adapted to be stepless movable, where a separation between adjacent tank sections remains intact during moving of the section walls.
It is therefore a purpose of the invention to provide a plant suitable for fish breeding, of the RAS type, which has a number of advantages over the current technology.